High power travelling wave tube having a negative circuiarly polarized electric field component



April 18, 1967 MULLER 3,315,118

HIGH POWER TRAVELLING WAVE TUBE HAVING A NEGATIVE CIRCULARLY POLARIZED ELECTRIC FIELD COMPONENT Filed April 25, 1962 2 Sheets-Sheet l Fig.7

R. MULLER April 18, 1967 A G N I m H Z T w HE P M A M W m; M L L I U E L C I L R F El VCC A I mm TIWUV T M A L W G E OE DLN H G I H Filed April 23, 1962 2 Sheets-Sheet 2 United States Patent 3,315,118 HIGH POWER TRAVELLING WAVE TUBE HAV- INC A NEGATIVE CIRCULARLY PGLARIZED ELECTRIC FIELD COMPONENT Rudolf Miiller, Strasslach, near Munich, Germany, assignor to Siemens & Halske Alrtiengesellschaft Berlin and Munich, a corporation of Germany Filed Apr. 23, 1962, Ser. No. 189,474 Claims priority, application Germany, Apr. 27, 1961, 5 73,696 3 Claims. (Cl. 3153.6)

The invention disclosed herein is concerned with a high power travelling wave tube for producing and amplifying highest frequencies, comprising a delay line along which is propagated an electromagnetic wave which enters into reciprocal action with an electron beam guided in a longitudinal homogeneous magnetic field.

In a travelling wave tube with delay line, the average electron velocity is as to magnitude and direction approximated to the phase velocity of an electromagnetic wave propagated along the delay line. A longitudinal magnetic field usually serves for guiding the electron beam. However, this magnetic field is not of direct importance for the high frequency mechanism of travelling wave tubes. The operation respectively with regard to amplification and excitation of the electromagnetic wave along the line is based upon the reciprocal action with the so-called slow space charge wave in the electron beam.

The advantage of the travelling wave tube resides in its broad band characteristic. The eiiiciency is however relatively low. The reason may be found in the velocity modulation of the electron beam effected by the reciprocal action between the electromagnetic wave and the space charge wave. It is due to this velocity modulation that the kinetic energy of the electron beam can be converted into high frequency only to a small percentage and that the kinetic energy remaining in the beam cannot be fully recovered. However, efiiciency is especially in connection with high capacity tubes of paramount importance.

It is accordingly the object of the invention to provide a travelling wave tube for producing and amplifying highest frequencies, especially a high power tube, wherein the reciprocal action between the signal wave and the electron beam does not cause a velocity modulation of the beam.

The invention makes use of knowledge obtained with respect to waves in an electron beam, which is in a longitudinal homogeneous magnetic field affected by transverse electrical field components of an electromagnetic wave propagated along a delay line. As contrasted with known travelling wave tubes, the longitudinal magnetic field is, in the presence oftransverse waves, of considerable importance for the high frequency behavior of the electron beam. It must be considered in this connection that fast and slow cyclotron waves form in the electron beam, owing to the deflecting action of the electrical transverse field with respect to the electron beam in the magnetic longitudinal field, whereby energy is supplied to the electron beam for the modulation of the fast wave, while energy is withdrawn from the electron beam for the modulation of the slow wave. The cyclotron wave is described by a rotation and simultaneous translation of the electrons of the electron beam, resulting in the field diagram, for the cyclotron wave, in a spiral line along which the electrons are propagated. The phase velocity of the cyclotron wave is respectively faster or slower than the average uniform velocity of the electrons, depending upon whether energy is supplied to or withdrawn from the electron beam incident to the modulation of such wave.

Two waves of a further wave type, the so-called synchronous waves, are formed in the electron beam in addition to the two cyclotron waves. The phase velocity of the two synchronous waves is always equal to the electron wave velocity, the designation of these waves as synchronous being traceable to this fact. The momentary image represents a spiral line just as in the case of the momentary image of the cyclotron wave. However, as contrasted with the cyclotron wave, the spiral line does not describe a rotation but only a translation. A model of such a wave would depict, for example, a garden hose which is circularly moved in parallel to the axis of the water stream. The two synchronous waves distinguish merely by their relative sense of rotation with respect to the cyclotron motion of the electrons. The wave with positive circular polarization (in the magnetic field in beam direction clockwise rotation) has positive energy, while the wave with negative polarization (counterclockwise rotation) has negative energy; that is, energy is incident to the modulation either supplied to or withdrawn from the electron beam. The same kinetic energy is thereby respectively supplied to or withdrawn from all electrons. Accordingly, upon modulating the electron beam solely with the negative circularly polarized synchronous wave, the beam will be slowed as a whole.

The invention proposes to achieve a high power tube, wherein the reciprocal action between the signal wave and the electron beam does not cause a velocity modulation of the beam, with utilization of the above described known appearance of the synchronous waves, in connection with a travelling wave tube of the initially defined type, by causing the delay line to carry a negative cir cularly polarized wave with a transverse electrical field component, the phase velocity of such wave being equal to the velocity of the electrons.

The beam as a whole is slowed up, owing to the reciprocal action between the negative circularly polarized wave on the delay line and the electron beam, and it is, accordingly, possible to match the phase velocity of the line wave to the velocity of the electrons, thus obtaining a high degree of electronic efficiency. Moreover, the kinetic energy remaining in the beam can be nearly com pletely recovered by the slowing up actionl Further details of the invention will appear from the description thereof which is rendered below with reference to the accompanying drawings, wherein identical parts are identically referenced.

FIG. 1 shows a linearly polarized electron beam which is modulated transversely, that is, cross-wise to its propagation direction;

FIG. 2 is a beam velocity-time or velocity-location diagram resulting in the beam operation according to FIG. 1;

FIG. 3 represents in diagrammatic manner an arrangement for withdrawing energy from the electron beam;

FIG. 4 is a velocity-time diagram illustrating the withdrawal of energy from the electron beam effected by the arrangement according to FIG. 3;

FIG. 5 illustrates part of a four-conductor helix operating as a delay line carrying a circularly polarized wave;

FIG. 6 shows a sectional view of the delay line of FIG. 5;

FIG. 7 represents an efliciency diagram of a travelling wave tube according to the invention; and

FIG. 8 is a velocity-time diagram of a velocity modu lated electron beam.

Referring now to FIG. 1, numeral 1 indicates the linearly polarized electron beam which is modulated in a transverse field. The electrons themselves, as in. the case of a synchronous wave, shall not have any transverse velocity. Numerals 2 and 3 indicate electrical parts, representing a short portion of a delay line. Accordingly, a coupling gap may be visualized as being formed between the parts 2 and 3. When the region 5 of the electron beam, moving in the direction ofthe arrow 4, is near the coupling gap, there will appear at the load resistor R a voltage with the indicated polarity. The electron beam is accordingly slowed up in the region 5. The electron velocity remains unaltered in the region 6, since such region is equally spaced from the parts 2 and 3. The electron beam is again slowed up at the region 7, the polarity at the coupling gap being opposite to that shown when the region 7 nears the gap.

There will therefore result a velocity distribution in the electron beam, according to the velocity-time or velocitylocation diagram shown in FIG. 2. The crosshatched areas indicate the energy withdrawal in the coupling gap between the parts 2 and 3.

The electron beam according to FIG. 1 shall be polarized circularly with a counterclockwise sense of rotation. Moreover, this circularly polarized beam shall permeate an arrangement adapted to also withdraw therefrom energy in a direction extending perpendicularly to the plane of the drawing FIG. 1. FIG. 3 shows a corresponding arrangement in sectional view, comprising electrically conductive parts 2, 3, 9 and 10 between which extends the spirally or helically formed beam 1.

The withdrawal of energy from the electron beam, effected by the arrangement according to FIG. 3, is illustrated in the velocity-time diagram presented in FIG. 4. The crosshatched area 11, corresponding to the area 8 in FIG. 2, represents the energy given off to the parts 2 and 3. Energy corresponding to the crosshatched area 12 is also given 011 to the parts 9 and 10. A superposition of the areas 11 and 12 results in a criss-cross hatched rectangular area 13, that is, the beam is slowed up as a whole.

It will accordingly be apparent that energy can be withdrawn from the electron beam, in a circularly polarized alternating field with transversal electrical field components, without effecting velocity modulation. The electron beam is thereby modulated with the synchronous wave which spreads exactly with the velocity of the electrons. The synchronous wave transports this positive or negative energy depending upon the direction of polarization thereof. (The example given in FIGS. 3 and 4 explains the wave with negative energy since energy is withdrawn from the beam.)

A four-conductor helix is adapted to operate as a delay line carrying a circularly polarized wave, FIG. 5 showing a part of such helix, comprising four conductors 14, 15, 16 and 17. This delay line shall be energized so that the electric field is formed in a plane perpendicular to the longitudinal helix axis, always between oppositely positioned conductors 14, 15 or 16, 17, respectively.

FIG. 6 shows the delay line of FIG. 5 in sectional view. The polarities indicated in FIG. 6 apply for a phase angle =0. Such a field distribution is obtained by feeding the signal wave to the pair of conductors 16 and 17 with a delay of a phase angle of 1r/ 2 as compared with feeding it to the pair of conductors 14 and 15. The input coupling line is for this purpose suitably branched and the electrical length of the two branches of the input coupling line are dimensioned so as to result in the desired phase difference.

IG. 7 is an efiiciency diagram of a travelling Wave tube according to the invention. The level 18 represents the kinetic beam power prior to entering into the delay line. The kinetic beam power of the electron beam is within the delay line converted into high frequency energy, so that the power falls from the level 18 to the level 19. The kinetc energy remaining in the beam between the delay line and the beam collector, at the level 19, can be recovered by the slowing up of the beam. Only a small part of the energy corresponding to the level 20 is lost as heat. The energy remaining in the beam could not be recovered to such extent in a previously known travelling wave tube, owing to the velocity modulation of the electron beam.

It will be seen from FIG. 8, which shows the velocity time diagram of a velocity-modulated electron beam, that a slowing up is possible only to an average velocity v along the line 21. A stronger slowing up would result in reflection of the slower electrons.

Changes may be made within the scope and spirit of the appended claims which define what is believed to be new and desired to have protected by Letters Patent.

I claim:

1. A travelling wave tube for producing and amplifying highest frequencies, comprising (a) a delay line,

(b) means for transmitting an electron beam longitudinally adjacent said delay line;

(c) means for propagating an electromagnetic wave along said delay line which enters into reciprocal action with the electron beam,

(d) means defining a longitudinal homogeneous magnetic field about said electron beam, and

(e) means for coupling a signal wave to said delay line to produce a predominantly negative circularly polarized electrical field component along said delay line, the rotation of said electrical field being in the same direction of rotation as that of said electron beam such that the phase velocity of said electrical field being equal to the velocity of the electrons.

2. A travelling wave tube according to claim 1, wherein said delay line includes a four-conductor helix, said helix being dimensioned and energized by said coupling means so that the electrical field is formed in a plane perpendicular to the helix axis, always between two oppositely disposed conductors of the helix.

3. A travelling wave tube according to claim 2, wherein the signal wave is applied to one pair of oppositely positioned conductors with a phase difference of 1r/2 as compared with the application thereof to the second pair of oppositely disposed conductors.

References Cited by the Examiner UNITED STATES PATENTS 4/1958 Dodds 3153.6 11/1958 Brewer 3l53.6 

1. A TRAVELLING WAVE TUBE FOR PRODUCING AND AMPLIFYING HIGHEST FREQUENCIES, COMPRISING (A) A DELAY LINE, (B) MEANS FOR TRANSMITTING AN ELECTRON BEAM LONGITUDINALLY ADJACENT SAID DELAY LINE; (C) MEANS FOR PROPAGATING AN ELECTROMAGNETIC WAVE ALONG SAID DELAY LINE WHICH ENTERS INTO RECIPROCAL ACTION WITH THE ELECTRON BEAM, (D) MEANS DEFINING A LONGITUDINAL HOMOGENEOUS MAGNETIC FIELD ABOUT SAID ELECTRON BEAM, AND (E) MEANS FOR COUPLING A SIGNAL WAVE TO SAID DELAY LINE TO PRODUCE A PREDOMINANTLY NEGATIVE CIRCULARLY POLARIZED ELECTRICAL FIELD COMPONENT ALONG SAID DELAY LINE, THE ROTATION OF SAID ELECTRICAL FIELD BEING IN THE SAME DIRECTION OF ROTATION AS THAT OF SAID ELECTRON BEAM SUCH THAT THE PHASE VELOCITY OF SAID ELECTRICAL FIELD BEING EQUAL TO THE VELOCITY OF THE ELECTRONS. 